(1) Field of the Invention
The invention relates to a method for measuring and evaluating energy consumption in electrical drive, said electrical drive including a control unit and an alternating current (AC) motor, in which method data of the state of the AC motor, such as power and frequency, is read and saved.
The invention also relates to an arrangement for realizing the method. In the arrangement for measuring and evaluating electrical drive energy consumption, said electrical drive includes a frequency converter and an AC motor, and said control unit includes a measurement unit for measuring the state data of the AC motor, such a power and frequency.
(2) Description of Related Art
At present, machines, devices and processes in general, particularly in the industrial sector, are used by electrical drives, where an essential element is an electric motor. Electrical drive is composed of a suitable electricity supply circuit, an electric motor and a control unit suited for controlling and/or regulating said motor. The machine or the like functions as the load in the electrical drive. The most common electric motor used in industrial processes is an alternating current motor, AC motor, particularly an induction motor.
Often the control unit employed in an AC motor is a frequency converter owing to the advantages achieved thereby. As an alternative, the electric motor can be realized as a direct current motor, DC motor, that is regulated by a suitable control unit. Another alternative is that the electric motor is not particularly regulated, but a separate regulation arrangement is made between the electric motor and the machine, or the regulation is realized as a procedure separate from the electrical drive in connection with the actual process.
Electrical drive is designed according to the process, machine or device that should be used by the electrical drive. The electric motor can be considered as a source of moment. The motor must be able to generate a certain moment, it must resist an overload of the process, but the motor must not be thermally overloaded.
Most advantageously an electrical drive provided with a frequency converter is operated by alternating current, AC. This kind of electrical drive comprises an electricity supply circuit, a frequency converter and an AC motor, advantageously an induction motor. A frequency converter includes a rectifier, a direct voltage intermediate circuit and an inverter. Through electricity supply, electric energy is obtained from an electric mains or a corresponding source of electricity, and the electric motor is run by means of said electric energy in order to drive an actuator, such as a machine or a device, connected to its axis. By means of the frequency converter, the electric motor is regulated, for instance as regards its speed of rotation and/or momentum, for driving the working machine and often also the process connected to said actuator.
The load in the electrical drive can be a machine running the process. Among these kind of machines, there are for example various pumps for transfer processes of liquids or the like, fans for air conditioning applications, various working machines, such as conveyors, feeders and machine tools, whereby the material belonging in the process is treated by transporting or by mechanically converting. At present, a common feature for nearly all processes is that they must be regulated and controlled.
There are many simple control methods and arrangements, such as throttling, bypass control and on-off control. The constructing of such arrangements is usually easy, and the investment in the control equipment may often look cost effective. However, simple control arrangements have remarkable drawbacks. An optimal process capacity, which ensures the best quality of the process, is extremely difficult to achieve with simple control arrangements. Moreover, it is pointed out that an increase in the production capacity generally requires reconstruction of the whole production process. Apart from this, with each direct on-line start-up, there is a risk of electrical and/or mechanical damage. An extremely remarkable drawback with simple control arrangements is that they consume a lot of energy, and energy is particularly consumed in various losses.
Electrical drives provided with a frequency converter are becoming more and more common in controlling industrial processes. Particularly such machines and devices where the speed control (speed of rotation) is an essential factor, have obtained electrical drive provided with a frequency converter. The employed frequency converter drives an AC motor, generally a squirrel cage motor, which means that mechanical control systems are no longer necessary. The speed of an electric motor is regulated by a frequency converter that converts the frequency of the voltage fed into the motor. The frequency converter itself is controlled with suitable electric control signals. Moreover, it is pointed out that through the frequency converter, an optimal quantity of electric power is fed in the electric motor and the process, and energy losses are thus avoided.
Four of the most common variable speed drives in the industrial sector are: mechanical variable speed control, hydraulic coupling, DC drive and frequency converter (AC drive). Mechanical variable speed control usually uses belt drives, and the controlling takes place by moving conical pulleys manually or with positioning motors. Hydraulic coupling applies the turbine principle, where the volume of oil in the coupling is changed, so that the speed difference between the driving and driven shafts changes. The oil amount is controlled with pumps and valves. In the DC drive, a DC converter changes the motor supply voltage fed to the DC motor in order to change the rotating speed. In the motor, a mechanical inverter, a commutator, changes direct current to alternating current. In the frequency converter, a standard squirrel cage motor is used. The speed of the motor is regulated by a frequency converter that changes the frequency of the voltage fed into the motor, as was already maintained above.
In mechanical and hydraulic variable speed drives, the control device is located between the electric motor and the working machine. This means that energy losses are created in the control device. Also the maintenance of the device is often difficult. In electrical drive provided with a frequency converter, i.e. electrical variable speed drive, VSD, all control systems are situated in an electric equipment room, and only the driving motor is in the process area. In many processes the production volumes change. Changing production volumes by mechanical means is usually very inefficient. With electrical drive provided with a frequency converter (i.e. electrical VSD), production volumes can be changed by changing the motor speed. This saves a lot of energy, particularly in pump and fan applications, because the shaft power of the motor is proportional to the flow rate to the power of three. Electrical VSD also has other advantages in comparison with electrical drives using conventional control methods and devices. We can use pumping as an example. In traditional methods, there is always a mechanical part and an electric part. In the electric part of throttling, there are needed fuses, contactors and reactors, and valves on the mechanical side. On-off control needs the same electrical components as throttling, as well as pressure tanks on the mechanical side. Mechanical parts are not needed in electrical drive provided with a frequency converter, because all control procedures are realized on the electrical side.
Many surveys and experiments have proved that with electrical VSD, there are easily achieved 50% energy savings in comparison with other conventional electrical drives and regulation systems. This means for example that if the power requirements of a constant speed motor and throttling control system would be 0.7 kW, with electrical VSD they would be 0.37 kW. If a pump would be used 4,000 hours per year, throttling control would need 3,000 kWh, and frequency converter drive would need 1,500 kWh energy per year. In order to calculate the savings, the energy consumption must be multiplied by the price of energy, which fluctuates depending on the availability and supplier of electric energy.
When comparing the advantages on one hand of a conventional constant speed electrical drive provided with a possible control arrangement connected therein, and on the other hand of electrical drive provided with a frequency converter, maintenance and upkeep expenses must also be taken into account. It has been estimated that in the maintenance of throttling control, there is consumed even ten times as much money as in the maintenance of a frequency converter drive. In many cases, a frequency converter needs hardly any maintenance at all, whereas the mechanical devices of conventional control systems require continuous maintenance.
Electrical drives provided with a frequency converter bring savings in energy in most applications, for example in industrial processes, which as such is a known an empirically proved fact. The problem is how to measure and/or evaluate energy consumption and particularly the achieved energy savings in comparison with other known electrical drives and control systems, particularly with electric motor drives operated at a constant speed and with process regulations connected thereto.
For a man skilled in the art, a known solution is to provide an existing, traditional electric motor and electrical drive with suitable measurement sensors, to measure power consumption at suitable intervals, and define energy consumptions on the basis of this. In that case there is measured the power and voltage fed in the electrical drive, and the power and energy consumption are defined according to this principle. On the other hand, as regards an electrical drive provided with a control unit, such as a frequency converter, it is possible to extract the data of the electric motor state, and the power and energy consumption can be defined on the basis thereof. A drawback of the measurement sensor arrangement is that the targets of measurement, i.e. particularly electric motors, must be provided with sensors. A drawback both in the measurement sensor arrangement and in the processing of measurement data extracted directly from the control unit is that both the sensors and the frequency converters must be connected by measurement wires to the measurement device always when measurements are carried out. Another drawback is that in this way, there is not obtained any direct comparison data for the energy consumption of various electrical drives, particularly for electric motors and working machines and the like, provided with various control arrangements. In practice, in one and the same target, the energy consumption of the electrical drive connected in the process is measured prior to replacing the drive (or when planning a replacement), and the energy consumption of a new electrical drive provided with a frequency converter is measured after replacing, i.e. after installing the electrical drive (or there are made preliminary calculations of its energy consumption), and the results are compared for proving that energy has been saved (or for estimating possible savings).
The following publications generally describe the state of the art: U.S. Pat. No. 3,998,093, US 2003/0057904A1 and WO 86/05887.
The patent publication U.S. Pat. No. 3,998,093 introduces a system for monitoring and controlling the consumption of energy, where the actual consumed energy is compared with an ideal or desired energy consumption, and both are displayed. The system is particularly used for monitoring the consumption of electric energy or gas in an industrial plant. The comparison between actual and ideal energy consumption is displayed for the working staff as visual information on a suitable display. When necessary, an alarm can be given, in case a predetermined threshold value between the actual and ideal energy consumption is surpassed. The object of this system is mainly to save energy, and particularly to monitor the ratio between the actual and the ideal energy consumption in an easily readable way.
The patent application US 2003/0057904A1 discloses a control system utilizing feedback, which system is applied in an electric motor running a rotary machine, such as a pump or a fan. This control system makes use of a predetermined specific curve of for example the capacity of a rotary pump and the motor intake power for realizing feedback control. This procedure replaces the earlier used pressure sensors or pressure difference sensors in the control system. The object of the invention is to optimize energy consumption and to reduce noise.
The international patent application WO 86/05887 discloses a power meter which is able to display the power consumed and the cost of power consumed. The device provides a continuous measurement of the electrical power consumed, which gives a more accurate measurement than measurement at discrete intervals of time.